1. Field of the Invention
The present invention relates to a magnetic position detector utilizing magnetic sensors, and in particular, relates to a magnetic position detector favorably used for a magnetic scale, a magnetic rotary encoder, and so on.
2. Description of the Related Art
As shown in FIG. 14, a magnetic position detector according to the related art includes magnetic member 801 and four spin-valve type magnetoresistive elements (SV-GMR1 to SV-GMR4). The magnetic member 801 is magnetized so that it alternately has north and south magnetic poles on its surface, and the SV-GMR1 to SV-GMR4 are at the same location with respect to an arrangement direction of magnetic poles of the magnetic member 801 (see the patent document, Japanese Patent Application Laid-Open No. 2006-23179). Magnetization directions of pinned layers (fixed magnetization layers) of one pair of the SV-GMR1 and the SV-GMR2 shift by 90 degrees from each other. Magnetization directions of pinned layers of the other pair of the SV-GMR3 and SV-GMR4 shift by 180 degrees from the one pair. Two-phase output signals (see FIG. 1B) whose phases shift by 90 degrees from each other are generated by a circuit with SV-GMR1 to SV-GMR4 moving in relation to the magnetic member 801, according to magnetic characteristic of a spin-valve type magnetoresistive element explained below.
As shown in FIG. 15A, a spin-valve type magnetoresistive element, for example a spin-valve giant magnetoresistive element (SV-GMR) includes a ferromagnetic pinned layer whose magnetization direction is fixed to one direction, a nonmagnetic layer through which electric current mainly passes, and a ferromagnetic free layer whose magnetization direction is same as a direction of an external magnetic field (external magnetic flux). When a magnetization direction of the pinned layer is same as a direction of an external magnetic field, resistance of the SV-GMR is low (low-resistance state “a” shown in FIG. 15B). As a direction of an external magnetic field rotates in a magnetic sensing surface of the SV-GMR, resistance of the SV-GMR changes according to an angle between the direction of the external magnetic field and the magnetization direction of the pinned layer. When the angle is 90 degrees, the external magnetic field causes no change in resistance so that the SV-GMR is middle-resistance state “b” shown in FIG. 15B. When the direction of the external magnetic field is opposed to the magnetization direction of the pinned layer, the SV-GMR is high-resistance state “c” shown in FIG. 15B.
FIG. 15C shows a magnetic characteristic in a magnetic sensing surface of the SV-GMR. The magnetic characteristic is shown as a relation of a resistance-change rate (ΔR/R) of the SV-GMR to an angle between a direction of an external magnetic field and a magnetization direction of a pinned layer, when the external magnetic field rotates around an axis perpendicular to the magnetic sensing surface (a plane which a free layer is in) of the SV-GMR. In this case, resistance-change rate (ΔR/R) changes sinusoidally.
In the magnetic position detector of the above patent document, a magnetic member is such that neighboring north and south magnetic poles have no space therebetween. The inventors have come to find a problem that in this case, as explained later in detail, a difference between a magnetic field in a stroke range of a magnetic sensor relative to the magnetic members and an ideal rotating magnetic field is large, so that accuracy of detection is not good.